//根据id获取节点  
function $(id){  
    return document.getElementById(id);  
}  
  
//根据id获取节点的值  
function $V(id){  
    return $(id).value;  
}  
  
//获取ajax对象  
function getXhr(){  
    var xhr = null;  
    if(window.XMLHttpRequest){  
        xhr = new XMLHttpRequest();  
    }else{  
        xhr = new ActiveXObject('Microsoft.XMLHttp')  
    }  
    return xhr;  
}  